The present invention relates to a method for substantially diminishing or essentially eliminating a visible knitline in formed plastic articles and to resinous compositions directed thereto.
Current trends in consumer products have created an unprecedented demand for plastic articles having unique color and surface appearance. For example, cellular telephones having interchangeable plastic covers are now commonly available. These covers come in a variety of colors and/or special surface effects, giving consumers control over the aesthetic design of their phones. Many other injection molded plastic products, such as, for example, computers, stereos and other consumer and/or business equipment, automotive interiors, etc., also benefit from having unique color and surface appearance. Plastic articles having a sparkling look, a metallescent or metal-like look, a true metal appearance, or any angular metameric appearance are desirable in many instances. Plastic articles having a sparkling or metallescent look can be created by incorporating a special visual effect additive such as free metal flakes into a plastic article at such loading that the individual flakes can be distinguished by the naked eye, resulting in an article with a sparkling or metallescent appearance. Plastic articles having a true metallic look can be created by using a much higher loading of free metal flakes. The drawback to products incorporating free metallic flakes is that the flakes have a high aspect ratio, and high reflectivity and therefore tend to change orientation at knitlines or other areas of non-uniform flow direction, aligning themselves with the flow field during processing, thereby causing visible flowlines, knitlines or weldlines and/or unacceptable variations in color or reflectivity in the finished product. Such flowlines, knitlines or weldlines produce unattractive streaks, marring the product's surface appearance. Thus, sparkling, metallescent and/or metal-looking plastic articles are often difficult to prepare by incorporating free metallic flakes into plastic articles.
There is presently no reliable method to ensure that pigments and additives having a high aspect ratio are optically anisotropic and rheologically isotropic when processed into plastic articles. Cubes and octahedra seem to satisfy these two conditions, and in fact, existing techniques have attempted to resolve the directionality problem of flake materials by encapsulating the flake materials in a substantially cubic material. While cubic shaped materials are less sensitive to flow-induced orientation than flakes are, and thereby reduce the appearance of flowlines, this technique does not eliminate flowlines. Additionally, producing particles of complicated geometries (i.e., other than spheres and platelets) is nontrivial, and introduces unnecessary filler mass to the final product. As a result, considerable efforts have focused on the investigation of processing changes to diminish or eliminate flowlines. In some instances, depending upon such factors as plastic article geometry, mold tooling can be modified to minimize the appearance of flowlines or to re-locate them to hidden portions of the plastic article. Sequential gating, the use of complicated mold temperature profiles, and agitation of the mold have been found to reduce the appearance of flowlines considerably. However, these methods are not always possible or successful, and all require the use of highly specialized equipment, making them uneconomical and impractical. Alternatively, using aluminum beads has been investigated as a possible solution. However, due to the absence of facets or sizeable reflective surfaces in aluminum beads, a reflective metallic look cannot be achieved in the end product. Thus, there is a need for systems and methods that ensure that the angular distribution of materials with high aspect ratios (for example, plate-like/flake pigments and additives) in complex flow fields remains optically anisotropic and rheologically isotropic during processing of plastic articles. There is also a need for such systems and methods to be less expensive and less highly specialized than existing systems and methods. There is yet a further need for such systems and methods to substantially diminish or eliminate the appearance of flowlines.
Foamed plastic articles containing special visual effect additives are not commonly available. One problem is that such additives are difficult to distribute uniformly throughout the foamed article, and special visual effects cannot be obtained. In addition, foamed articles containing such additives typically have poor surface quality.